Military, government and some commercial institutions rely on secure voice transmission systems to conduct their business. This is usually achieved by encrypting communications and providing subscribers with cryptographic devices that can encode transmitted signals and decode received signals. These devices must work over the existing telecommunications infrastructure and therefore are subjected to substantially different quality transmission paths dependent upon the subscribers' locations. For this reason different cryptographic devices are used to provide secure communication over different routes. Unfortunately these different cryptographic devices cannot communicate directly with each other and subscribers may thus need to be equipped with several devices, having to select and connect the appropriate one in order to achieve secure voice communications.
One approach to this is to provide a cryptographic gateway that permits different cryptographic devices to interoperate. This provides a seamless communication channel between two subscribers with different cryptographic devices. Cryptographic interworking is achieved by essentially connecting two different systems back-to-back and this can be controlled manually, semi-automatically or fully-automatically. Manual gateways are expensive to run, especially if 24 hour service is required. Previous designs of automated gateways have required bespoke developments making them relatively expensive. In one automatic gateway cryptographic pairs are associated with a dedicated PABX, the PABX provided with extra functions and additional control equipment specifically to support the gateway facility.
In addition, a number of cryptographic pools employing trunks have been proposed for PABXs. In Japanese Patent Application number 85-121742 (published as JP 61-278256) a PABX is provided with a crypto trunk. A subscriber requiring encryption dials the number of the crypto trunk and the call is routed through the crypto trunk. This arrangement requires modifications to the PABX, and neither provides a selection of crypto types nor the facility for cryptographic conversion. In U.S. Pat. No. 5,115,466 a crypto-pool is connected to a PABX by a trunk. In the crypto-pool one crypto device is provided for every pair of trunk lines, and a management unit coordinates operation of the crypto-pool. This has the disadvantage of the cost of the management equipment, and does not provide for cryptographic conversion. This arrangement also requires authentication and the exchange of crypto keys. Patent Application EP 511,497 A2 (Alcatel) discloses a security guard which includes crypto modules, routing devices and a managing device for managing the crypto modules.
The object of the invention is to provide an improved cryptographic interface between subscribers which minimises or overcomes these disadvantages.